The present invention relates to a method and a device for correcting the trajectory of spin-stabilised projectiles such as artillery shells.
The current trend in artillery technology is to try to extend maximum range while also minimising time at the deployment site after opening fire. The latter is because there are very good methods for rapidly and accurately detecting the deployment site of an artillery gun that is firing or has just fired. In other words, there is a major risk of quick counter fire. One method to minimise the time at the deployment site is to have the required number of shells in the air before the first shell has reached the target. This tactic restricts the opportunity to first fire one or more ranging rounds and subsequently correct elevation and traverse on the basis of their impact points or true position when well out in their trajectory.
The method briefly mentioned above of firing a number of rounds into the air before the first one has reached the target requires a method for correcting the trajectories of the shells other than by fire-based criteria such as muzzle velocity, elevation and traverse.
Since it is generally acknowledged in the field of artillery that longitudinal dispersion is usually greater than lateral dispersion it is natural that initial efforts focused on developing methods for correcting the trajectory longitudinally while the projectiles are still airborne.
Such a method and its associated device are described in Swedish Patent No. 8301651-9 (=U.S. Pat. No. 4,655,411). According to this patent target position is measured, the muzzle velocity of the ammunition unit (the shell) is determined, its position and velocity in trajectory are possibly checked, and on the basis of these values the probable impact point longitudinally, and the deceleration necessary to achieve the correct impact point longitudinally, are computed. A point in time is also computed for activating the available deceleration device to achieve the necessary deceleration to provide the correct range. This procedure thus presupposes that the gun elevation and muzzle velocity selected result in a range that is somewhat too long. It is proposed to achieve the deceleration thus required either by deployable deceleration devices or by blasting parts off the nose cap of the shell to increase its air resistance. Thus to function in the way outlined above, besides being equipped with a deceleration device the shell must also contain a receiver for receiving the necessary commands for trajectory correction and for initiating the deceleration device at the right point in time. Of course, a corresponding technology could be used with a rocket-assisted projectile or shell equipped with base bleed in which the receiver indicated above would initiate the igniter of these devices to achieve the desired increase in range.
As already mentioned no corresponding work has been done as far as we know to develop an elementary method that would function in field conditions for lateral correction of spin-stabilised shells while they are still in trajectory. The purpose of the present invention is to offer such a system.
It has long been known that an aerodynamic effect gives each spin-stabilised projectile a clear tendency to drift laterally from the course imparted to it at launch. The direction in which the projectile drifts is completely dependent on its own direction of spin, while the magnitude of the drift is dependent on the rate of spin of the projectile.
On canons, howitzers, and other gun barrel weapons the rifling in the barrel is traditionally almost always clockwise, i.e. like a standard screw thread but with a completely different pitch. The clockwise spinning shell now has a tendency to drift to the right, i.e. in the same direction as it rotates, compared with the direction it was given when fired. The lateral error that thus arises is well-known, and firing tables used in the field regularly contain corrections to eliminate its effect on the firing result. As already pointed out the range of artillery has been enhanced significantly in recent years while the capability of tracking the trajectory of one""s own shells has also been improved. The lateral error built into spin-stabilised shells thus becomes more pronounced as it increases with range, while the capability for stating the trajectory corrections needed is also improving all the time.
As claimed in the present invention it is proposed that one does not only correct for the known aerodynamic effect by a compensation in traverse when firing, but instead exploit this phenomenon for an active correction of the shell""s trajectory while the shell is en route to the target.
The most elementary version of the present invention ought to be to have slightly less aim-off to the side of the target than specified in applicable firing tables, and then subsequently when measuring the position of the projectile in trajectory to compute what deceleration of the rate of spin of the projectile is required to enable a hit in the target.
Deceleration of the projectile""s rate of spin can be achieved by deployable devices, but as this concerns reducing the rate of spinxe2x80x94not trajectory velocityxe2x80x94the deceleration devices will be of a different design from those indicated in the above mentioned Swedish Patent No. 8301651-9. On the other hand, the method in the present invention for correction of the projectile""s trajectory laterally may very well be combined with the method described in that patent for a correction of the same projectile""s trajectory longitudinally.
If one combines both these methods it then becomes possible to correct the trajectory of a shell both laterally and longitudinally. The proviso for this is primarily that the shell is equipped with spin and velocity decelerators, and that the shell receiver can receive and initiate deployment of these devices individually at the points in time calculated to provide the desired trajectory correction laterally and longitudinally.
Since the method for measuring the position of a projectile in its ballistic trajectory and the method for computing its point of impact from the value thus obtained, together with the muzzle velocity of the projectile and the training of the barrel in elevation and traverse are well known, they will not be described in any further detail herein.
However, it may be justified to mention that in our own Swedish Patent No. 8600649-1 there is a description of a method and a device involving a shell being equipped with spin retarding devices. At first glance this patent may appear to contain similar concepts to those of the present invention, but in the earlier patent the objective is not to achieve a trajectory correction but merely to adjust the gyro stability of the shell to the actual ballistic trajectory in which it was fired. It is thus a method to reduce dispersion and is not a method for active trajectory correction.